In the control of internal combustion engines, the conventional practice utilizes electronic control units having volatile and non-volatile memory, input and output driver circuitry, and a processor capable of executing a stored instruction set, to control the various functions of the engine and its associated systems. A particular electronic control unit communicates with numerous sensors, actuators, and other electronic control units necessary to control various functions, which may include various aspects of engine malfunction diagnostics, fuel delivery, transmission control, or many others.
Malfunction diagnosis in internal combustion engines is commonplace. This is due to the desire to detect engine failures or malfunctions before they occur, or as soon as possible after they occur, to provide an opportunity for remedial measures to avert severe engine damage. Crankcase pressure has been used to detect various piston-related engine malfunctions. One method is to use a pressure limit switch which provides a signal when crankcase pressure exceeds a predetermined threshold. This approach requires a pressure limit switch selected based on a particular application or an engine family which exhibits similar crankcase pressures during operation. A crankcase pressure exceeding the predetermined threshold results in the limit switch generating a signal which is used by the electronic control unit to indicate a fault. The electronic control unit may then take appropriate action depending upon the particular application, which may include generating a warning message or indicator light, or shutting down the engine, for example.
Another method of detecting an engine malfunction based on crankcase pressure uses a pressure sensor which provides a continuously variable signal representing current crankcase pressure. The electronic control unit periodically samples the signal generated by the pressure sensor and compares it to a predetermined crankcase pressure limit. The pressure limit is generally a value which is determined based on expected crankcase pressure at the maximum rated engine speed and load (or power). This method allows the use of a single crankcase pressure sensor for a variety of applications since the limit value may be calibrated for each engine or engine family and stored in the electronic control unit. While this strategy is capable of detecting various piston or cylinder-related malfunctions at maximum speed and load, it is insufficient to detect impending failures at lower speeds and loads. In addition, it is desirable to detect failures or impending failures as quickly as possible to provide the electronic control unit or the vehicle operator sufficient time to take remedial actions to avoid a catastrophic failure.